1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery.
2. Description of the Related Art
Recently, with the current trend in downsizing electronic appliances such as mobile communication appliances, notebook computers, palmtop computers, camcorders, portable CD (MD) players, and cordless telephones, there is an increasing demand for batteries of smaller size and larger capacity as power sources for these electronic appliances.
Batteries widely used as power sources for these electronic appliances include primary batteries, such as an alkaline manganese battery, and secondary batteries such as a nickel cadmium battery and a lead battery. Among others, a nonaqueous electrolyte secondary battery comprising lithium composite oxide as a positive electrode active material and a carbonaceous material (graphitized material or carbon material) capable of intercalating and deintercalating lithium ions as a negative electrode active material has attracted attention because of its small size, light weight, high cell voltage, and high energy density.
As the negative electrode, instead of the carbonaceous material, lithium or lithium alloy can be used. In such a case, however, as the secondary battery repeats charging and discharging, dissolution and deposition of lithium are repeated, and an acicularly grown dendrite is formed, and as the dendrite penetrates through the separator, an internal short-circuit may occur. On other hand, the negative electrode comprising a carbonaceous material can suppress the growth of dendrites as compared with the negative electrode containing lithium or lithium alloy.
On the other hand, as a case for containing an electrode group including the positive electrode and negative electrode as described above, a laminate film obtained by laminating a metal foil such as aluminum and resin is formed like a bag or cup, and the nonaqueous electrolyte secondary battery is further reduced in weight and size.
As the nonaqueous electrolyte, a nonaqueous electrolyte comprising cyclic carbonate such as propylene carbonate (PC) or ethylene carbonate (EC) as a main solvent is known.
However, the nonaqueous electrolyte secondary battery comprising a nonaqueous electrolyte using a nonaqueous solvent including PC and EC generates gas as the nonaqueous solvent is decomposed at the time of initial charging, and the charging and discharging reaction is disturbed by this gas, which makes the initial charging and discharging efficiency poor. Also in the nonaqueous electrolyte secondary battery using such nonaqueous solvent, if a highly flexible laminate film case is used, the case may be deformed significantly due to gas generated in initial charging. Swelling of the case brings about various problems such as oversize of a battery in an electronic appliance and malfunction of an electronic appliance.